An apparatus of this kind is known from U.S. Pat. No. 3,568,372.
The apparatuses for checking features of parts, for example the apparatuses for checking linear dimensions and geometrical and shape features of parts, must sometimes be provided with means for enabling the mutual fitting between the part and the sensing means, for guaranteeing the proper positioning of the sensing means and consequently the proper checking.
A typical case in which this need arises relates to the automatic dynamical checking of linear dimensions and/or geometrical and/or shape features of crankpins of crankshafts. The crankshaft is located on the apparatus, for example on Vee rests or between two centres, so as to enable its rotation about the geometrical axis defined by the main journals.
Then the sensing means, constituted in particular by comparator gauging heads with contact feelers for checking linear dimensions, are displaced into cooperation with the crankpins and the checkings are performed in dynamical conditions. Of course, since the crankpins rotate eccentrically with respect to the rotation axis of the crankshaft, it is necessary that the sensing means may move with respect to the frame or bed of the apparatus, for following said eccentric movement.
For this end, in accordance with U.S. Pat. No. 4,351,115, the sensing means are coupled to Vee reference and support devices, that cooperate with the surfaces of the crankpins, and each Vee reference and support device is carried by a connection device including two legs coupled to each other by a cylindrical hinge. Another cylindrical hinge couples the end of one of the legs to the apparatus frame. During rotation of the crankshaft, that is horizontally arranged, the two hinges enable the reference and support devices and the sensing means to follow the eccentric movement of the crankpins. The reference and support devices and the connection devices are arranged above the part and the reference and support devices rest on the crankpins due to the forces of gravity.
A drawback of this apparatus, which is particularly grave when it is desired to perform combined checkings, for example checking of the parallelism among the geometrical axes of the main journals and those of the crankpins, derives from the use of cylindrical hinges obtained through ball bearings. In order to limit backlash and friction, it is necessary to use pre-loaded precision ball bearings and frequently perform their maintenance. This implies high costs and in any case, since it is not possible to fully eliminate plays and friction, the precision and repeatability of some checkings are poor.
In the above mentioned U.S. Pat. No. 3,568,372 there is described, among other things, an electronic comparator gauge for external cylindrical grinders comprising a reference and supporting device with two shoes arranged in a V configuration, a feeler horizontally movable along the V bisecting line and a device for supporting the feeler and a corresponding transducer, as well as the reference and supporting device.
The device supporting the feeler is housed with radial play within a conical seat of a movable support constituted by a slide actuated by a hydraulic cylinder and can be advanced, with respect to the slide, by a stepping motor, for keeping the shoes into contact with the part during the grinding operation. Both the shoes are maintained into contact due to connection means comprising two leaf springs having the ends respectively locked to the device supporting the feeler and to the slide. In rest conditions the springs substantially take the shape of half cylinders and can undergo resilient deformations to maintain said contact. An excessive deformation of the springs can be detected through a strain gauge transducer fixed to one of the springs.
This arrangement only permits small displacements of the shoes and the gauge, as a whole, is not adapted to check sections of a part rotating with considerable eccentricity about a main rotation axis of the part.